1. Field of the Invention
The present invention relates to an apparatus for applying a viscous liquid material, in particular, to an apparatus for injecting a viscous liquid material such as an adhesive with following a predetermined track on a surface of an object to be applied, and relates to a method of controlling application in an application apparatus.
2. Description of the Prior Art
Conventionally, work steps of applying a viscous liquid material such as an adhesive and the like have been utilized in various manufacturing processes in such a way to draw a line or a band with the viscous material on a surface to be applied, following a predetermined track. Specifically, in a process for assembling automobiles, a liquid rubber called a sealant is applied to the periphery of a sheet glass, and it is then pressed to a window panel at a normal temperature, thereby the sheet glass is adhered to the window. For another example, in assembly of picture tubes for a television set, a frit is applied to a joint surface at the brim of a coned funnel, and a front grass is then mounted on the brim of the coned funnel to be adhered by baking at a temperature of about 450.degree. C. As illustrated above, the apparatus for applying a viscous liquid material have been widely utilized in the industrial field.
The apparatuses which have been conventionally utilized for applying a viscous liquid material can be generally classified into three types, that is, a valved-tank type, a type of having a pressured tank and a separate valve, and a type of pumping to a separate valve.
FIG. 1 shows a construction of a valved-tank type frit application apparatus. In the frit application apparatus of FIG. 1, a large tank 1 containing a frit, whose capacity is rather large, for example, about 2 liters, is located with a robot hand (not shown) above a funnel 2 whose surface to be applied is faced upwards toward the tank 1. The tank 1 has a nozzle 3 at a bottom end thereof, and the tank 1 is moved by the robot hand so that the nozzle 3 traces on an application track, that is, the brim of the funnel. Inside the tank 1, a needle valve 4 is arranged to be axially moved downwards by a piston device 5, so that a tip end of the needle valve seats on a valve seat 4a to close the nozzle 3. Moreover, a blade 7 which is driven by an actuator 8 is provided inside the tank 1 in order to agitate the frit. On the other hand, a tube is provided on the tank 1 in order to apply an air pressure to inside of the tank 1 at a preset level of 0.2 to 0.5 kg/cm.sup.2. A sensor 9 is provided for detecting a initial point at which a band of the frit applied on the funnel begins on the surface.
According to the above construction, the needle valve 4 opens the nozzle 3 to apply the frit to the brim, while it travels along the brim of the funnel. Near the termination of the application along the brim, the initial portion of the frit band is detected by the sensor 9, and the needle valve 4 is then closed so that application of the frit is stopped at the initial point, thereby achieving a cycle of frit application.
However, in the apparatus described above, the tank 1 has large weight and high inertia. Therefore, correctness of the operation of the tank 1 by using the robot arm tends to decrease. Moreover, it is very difficult to control a volume of the injected frit by regulating the needle valve 4 which is moved downwards at the termination of the application in response to detection of the initial portion of the applied frit band. In particular, at closing of the needle valve 4, the frit between the needle valve 4 and the valve seat 6 is forced out of the nozzle 3 by the needle valve 4, so that the band drawn with the applied frit is suddenly swelled at a site A ahead the terminal point, which should meet the initial point, as shown in FIG. 2, and the band becomes extremely poor just behind the site A. Therefore, uniformity of the frit band deteriorates near the terminal point. Accordingly, it may cause incomplete adherence between the glass and the funnel assembled, which results in production of sub-standard articles, low reliability of the product and the like.
FIG. 3 shows a construction of a frit application apparatus of a type of having a pressured tank and a separate valve. In this apparatus, a frit which is enclosed in a main tank 10 and agitated with a blade 7 rotatively driven by an actuator 6 is pressed by an air pressure to be fed through a gate valve 11 and a flexible connecting tube 12 to a valve device 13 which is provided separately from the main tank 10. The valve device 13 is arranged to be moved by a robot arm (not shown) and the like above a surface to be applied of the funnel 2. In accordance with the above construction, the valve device 13 travels on an application track, while the valve is operated to open, thereby achieving frit application.
However, in this apparatus, since the frit is fed through the flexible connecting tube 12, the pressure applied on the frit decreases from a value Pt at the tank 10 down to a value Pi at an injection port of the valve device 13 according to the distance from the tank 10 as shown in FIG. 4. Therefore, it is rather difficult to control the injection pressure Pi. In addition, since the injection pressure Pi also easily changes in response to the travelling movement of the valve device 13, the frit applied to the funnel forms an uneven band 14 as shown in FIG. 5. Moreover, according to sudden pressure change at the gate valve 11, some component blended in the frit sometimes separates from the frit in the connecting tube 12, etc., the frit cannot be applied, accordingly, in a normal condition.
FIG. 6 shows a construction of a frit application apparatus of a type of pumping to a separate valve. In this apparatus, in place of a gate valve 12 of the apparatus shown in FIG. 3, a screw pump 15 is utilized for forcingly feeding the frit to the valve device 12. Therefore, this apparatus can avoid the frit separation caused by pressure decrease at the gate valve illustrated above. However, since the frit feeding method of this apparatus is basically the same as that of the apparatus shown in FIG. 3, the injection pressure at the valve device 13 cannot be controlled to fluctuate unstably. Therefore, the frit applied on the funnel 2 similarly draws a non-uniform band such as shown in FIG. 5.
Of course, in a process for applying a sealant to a sheet glass, the same problems as illustrated above similarly arises as frequently as in that for a frit application to a funnel described above.
Consequently, the conventional apparatuses for applying a viscous liquid material have various problems to be solved such as incorrect positioning due to weight and inertia of a large-volume tank, difficulty in control of an applied volume, unevenness of the band traced with the applied liquid material, component separation of the frit, etc. In other words, it is difficult to apply a viscous liquid material uniformly with high reliability.